With the advent of electronic typewriters and computers and the need for multiple copy forms packs, attention has focused on the rapid recording of information on forms packs with a variety of printers. While it is possible to produce forms packs on which information can be recorded through multiple "parts" (plies) of carbonless paper with heavy-impact printers, many electronic typewriters and printers associated with computer keyboards are light-impact in the sense that the printing mechanisms utilize light force marking means. Typical of such typewriters and computer actuated printers are dot matrix and daisy wheel printers. These printers are effective for data entry on carbonless forms packs of three or four parts, and in some cases even five part forms, but clarity on the last (bottom) parts is diminished when the forms packs contain six or more parts, and data becomes illegible when eight or more copies are desired.
Another problem is the scannability of bar codes on the individual sheets of a carbonless forms pack. It has not been possible, heretofore, to obtain sufficient quality of transfer of bar codes, even in only three part packs, such that all sheets in the pack are scannable. For this reason, bar codes have been pre-printed on the individual sheets of form packs. Obviously, it would be advantageous to be able to produce carbonless multi-part forms packs where bar codes as well as other indicia can be transferred with clarity to all sheets of the pack.
Carbonless paper is produced commercially by coating a paper sheet with a composition, preferably a hot melt composition, containing a chromogenic material which develops an image when the sheet is contacted with a marking force. Marking forces may be either manual (such as the pressure exerted by a pen, pencil or other hand-held stylus) or machine actuated (such as the styli or characters of dot matrix or daisy wheel printers). Chromogenic materials comprise color precursors, color formers, color receptors, color inhibitors and other image-generating substances, including mixtures of two or more thereof, which interact chemically to develop an image. Generally, a chromogenic material is encapsulated or otherwise protected by a film or other material which is ruptured by the marking force to cause contact of the chromogenic material with air or other actuating (developing) medium to induce the color change defining the image.
Typically, a chromogenic material comprises a microencapsulated color precursor either alone or suspended in a liquid color receptor (developer). When the color precursor only is in the coating composition, a second coating composition containing the color receptor is applied over the first coating, or the color receptor coating is applied to the obverse side of the sheet or to another sheet superposed on the sheet coated with the color precursor. It is also possible to encapsulate the color receptor and maintain the color precursor as the continuous phase in the coating (on the same surface as the color receptor or on the surface of another sheet superposed on the sheet coated with the color receptor). Also, each of the color precursor and color receptor may be encapsulated. The interactive color forming reagents may also be isolated from one another (until brought into contact by a marking force) by other means such as an intervening rupturable sheet or film.
In one common form of forms pack, the top sheet (called the "record" sheet because it receives the marking force directly) is coated with a composition containing a color receptor. This coating is termed a "CF" coating. The bottom sheet in the pack is coated with a composition, preferably a hot melt composition, containing microencapsulated color precursor material. This coating is called a "CB" coating. The intervening sheets in the pack are coated on one surface with the CF coating and on the obverse surface with the CB coating. These sheets are therefore called "CFB" sheets. Both the bottom sheet and the intervening sheets ar called "transfer" sheets.
In another embodiment of forms packs, a single composition, comprising an encapsulated color precursor dispersed in a continuous phase containing a color developer, is coated on the front surface of a paper sheet, thus defining a transfer sheet. Multiples of such transfer sheets are superposed under a single, uncoated, top record sheet to form copies of the markings imposed on the record sheet. These packs are said to be "self-contained" because both of the interactive materials required to form an image of a marking are in the same coating.
Representative hot melt coating processes preferably used for the production of pressure sensitive carbonless tranfer sheets are disclosed in U.S. Pat. Nos. 4,063,754, 4,137,343, 4,139,218, 4,139,392, 4,143,890, 4,203,619, 4,336,067, 4,112,138 and 4,097,619. The foregoing patents (the disclosures of which are incorporated herein by reference) not only describe hot melt coating compositions and systems for the production of carbonless transfer sheets and forms packs, but also a variety of chromogenic materials, both encapsulated and non-encapsulated, useful in the production of carbonless paper.
Accordingly, there exists a critical need for a pressure sensitive carbonless transfer sheet capable of providing from light impact printers, clear copies throughout forms packs containing at least six of the transfer sheets, and more usually eight or more parts such as the 8-12 part forms now in demand by government agencies and industry.